The present invention generally relates to artificial lift systems which are utilized for production of fluids from subsurface reservoirs, including oil, water, and liquid phase hydrocarbons. More particularly, the present invention is utilized with artificial lift systems where a subsurface pump is actuated by a plurality of rods connected end-to-end, herein collectively referred to as a “rod string.” The rod string is set within a plurality of tubing joints likewise connected end-to-end, wherein the tubing joints are collectively referred to as a “tubing string.” Actuation of the subsurface pump lifts the fluid upwardly from the subsurface pump to the surface, where the fluids flow in the annular space between the rod string and the inside diameter of the tubing string as the fluid flows upwardly.
There are generally three different types of subsurface pumps which can be actuated by rod strings, two which are positive displacement pumps, and the third is a roto-dynamic pump. The first type of subsurface pump is a plunger/barrel pump, wherein the reciprocal motion of the rod string reciprocates the plunger within the pump barrel, such that liquids are drawn into the pump barrel and lifted upwardly through the tubing string. The reciprocating motion of the rod string is typically imparted by a pump jack. The second type is a progressive cavity pump, wherein the rotational motion of the rod string operates a rotor which turns within a stator, and fluid is transferred by means of the progress through the pump of a sequence of small fixed shape cavities as the rotor is turned. In these systems, the rod string is rotated by a motor/gear reducer combination installed at the surface. The third type of pump, the roto-dynamic pump, is a centrifugal pump in which the rod string rotates a series of impellers. This type of system utilizes a similar motor/gear reducer combination as utilized for the progressive cavity system.
For all of these systems, the uppermost rod in the rod string is a polish rod. The polished rod reciprocates in and out of, or rotates within, a stuffing box. The stuffing box is a close-fit assembly which cleans the polished rod, prevents debris from entering or exiting the well, and further prevents fluid from leaking from the well during operation. The stuffing box is typically mounted above a T-fitting at the top of the tubing. The stuffing box provides a dynamic seal along the length the polish rod. The stuffing box typically has a central passage through which the polish rod moves, while stuffing or packing material is compressed by an enclosing cap or fitting of the stuffing box which urges the packing material against the sides of the polish rod to create fluid seal. The packing materials are typically elastomers and other materials which are softer than the polish rod material.
The movement of the polish rod within the packing material generates friction, and thus heat, which breaks down and degrades the packing materials. This process reduces the integrity of the seal formed between the packing material and the polish rod. In further aggravation, the presence of solids in the produced fluid, such as sand, can accelerate this degrading of the packing material as well as adversely impact the life of the polish rod. The loss of the integrity of the seal between the polish rod and the packing material will result in the escape of fluids from the well which can result in environmental damage and the loss of valuable resources, and can result in significant clean-up expense and potential fines and penalties. Accelerated packing replacement also requires the expenditure of man-hours which might otherwise be avoided.
The lubrication of the polish rod and the packing material reduces the friction, and thus the heat, generated between the polish rod and packing material. This lubrication reduces the wear caused by movement of the polish rod within the packing material and extends the life of the packing and the polish rod. The typical lubricant used for this service is grease, which is typically placed in the interior of the stuffing box and carried by the polish rod into contact with the packing material. There are various proprietary lubricating greases which work particularly well for this service, where the greases may have enhanced mechanical and thermal stability, resistance to water, and corrosion resistance additives. Application of the lubricant is typically done by hand or by mechanically operated systems which continually apply a small dose of grease according to the motion of the walking beam of the pump jack, which may be connected with linkage or cable to a mechanical pump which strokes with the motion of the walking beam. While preferable to no lubrication at all, the manual lubrication requires the use of personnel, and can be overlooked and inconsistently applied. The mechanically operated systems are subject to failures in mechanical linkages, often have limited grease reservoirs, and may not deliver a consistent volume of grease into the stuffing box.